The present invention relates broadly to jet engines, and in particular to a band clamp apparatus to allow an engine change without removal of the exhaust system.
The state of the art of engine band clamps is well represented and illustrated to some degree by the prior art apparatus and approaches which are presented by the following U.S. Patents:
U.S. Pat. No. 2,720,079 issued to Mines on Oct. 11 1955;
U.S. Pat. No. 2,773,709 issued to Smith on Dec. 11 1956;
U.S. Pat. No. 2,839,894 issued to Shutts et al on June 24 1958;
U.S. Pat. No. 3,903,693 issued to Fox on Sept. 9 1975; and
U.S. Pat. No. 4,480,436 issued to Maclin on Nov. 6 1984.
The Mines patent describes a clamp which is fashioned in several parts of different materials whereby a pair of conduits of one material may be clamped together by a clamp of different materials which will expand and contract substantially equally with the conduits during changes in temperature thereby resulting in a uniformly tight joint at all times.
The Smith patent is directed to a bimetallic clamp which has indentations on the inner face of the band in each of which the free legs of one or a pair of bimetallic means or retainers rest, the indentation acting as a stop to limit retraction of the retainers under temperature change and to give the necessary pressure on the flanges with a smaller change in band diameter.
The Shutts et al patent discloses a gas turbine combustion chamber which utilizes a split-ring clamp that is bolted together, to secure a flanged end cover to the complementary flanged end of the air jacket of the combustion chamber.
The Fox patent discusses a rocket motor housing in which a trailing segment or nozzle is attached to a leading segment by an annular retaining ring. The ring comprises a central hoop segment with a pair of radially inwardly directed flanges. A pair of outwardly extending flanges on the ring receive a locking screw. A gasket ring is also shown in the Fox patent.
The Maclin patent shows a gas turbine engine construction which includes a turbine nozzle stage that is connected to the outlet of a combustion zone by bolts. The bolts pass through abutting flanges on a frame and the nozzle stage.
The designers of modern high performance aircraft require that the contours of exhaust nozzles blend well with the aircraft for low afterbody drag and high performance. In addition, the exhaust system which may include jet deflection for flight maneuver and thrust reversal capability as well as nozzle area variation, is increasingly being integrated into the aircraft structure. Both common use of structural members by the aircraft and engine exhaust system results in a lower overall aircraft system weight when compared to a conventional design wherein engine and aircraft have entirely independent structural members. Further trends toward exhaust system integration arises from the extension of the exhaust system function from the conventional engine exhaust area variation to flight maneuver and reversing as required on advanced aircraft. These requirements demand that exhaust control and actuation be accomplished directly from the aircraft, and be essentially independent of the engine control system.
The needs of low drag contouring and structural sharing by the aircraft and the exhaust system when combined with aircraft control of flight maneuver functions of the exhaust systems, results in an exhaust system which is generally difficult and time consuming to remove during engine maintenance. This fact together with the concurrent emphasis on very quick engine replacement as assurance that aircraft may be operational near full time, dictates that the propulsion system include a new means of attaching the engine to the exhaust system. The conventional bolted flange not only requires far too much maintenance time due to the number of bolts involved (50 to 100 bolts) but when installed, the only bolts that are accessible, are those on the bottom of the engine. There are no access doors on top of typical aircraft.
However, the nature of the loading at the engine-exhaust system interface poses a number of difficult problems in designing a quick maintenance clamp for attaching engine and exhaust system.
1. The pressure load tending to separate the joint is very high.
2. The separating loads are often non uniform, circumferentially, due to the close proximity of the engine mount and also to bending moments which result from flight maneuvers. This effect is particularly severe for any band clamp since a local high load can, due to local rolling or twisting of the flanges, cause the band to be displaced radially outward with risk of disengagement.
3. High shear load is applied due to aircraft maneuvers.
4. High torque must be transmitted across the joint to the engine mount should an inadvertent hard rub or seizure condition occur in the rotating machinery.
5. On rapid engine start or burst conditions the flanges will be higher in temperature than any external clamping band. Consequent thermal straining of the band may well be plastic and can result in loosening of the joint or rupture of the band.
6. Very high reliability of the clamping means is required since failure can result in not only power loss but severe damage to the aircraft and engine.
7. Replacement of engine in the field must be simple and quick with access only from the bottom.
The only type of joining means which meets the maintenance turn around time requirement is a band type clamp since few bolts are required and a band clamp is consistent with the limited, bottom only, access.
The well known conventional V-band clamp will solve item 7 very well, is in fair agreement with items 1 and 3 and can handle item 4 of torque provisions are added.
The critical items 2, 5 and 6 however, are not satisfied at all by the prior art conventional V-band clamp.
The previous prior art attempts to solve the problem had focused on the simplicity and maintenance advantages of the conventional V-band clamp which were used in attaching exhaust duct to aircraft engines. These results have been generally unsatisfactory and these clamp devices are no longer used on high temperature engine duct connections. The typical difficulties that were experienced are an exhaust nozzle lost in flight on IA engine and more recently a helicopter exhaust pipe lost in flight.
The conventional V-band clamp has the following limitations:
(1) it depends on high band tension to hold joint together. This is intolerant of any temperature difference between the flanges and the band since the highly stretched band may yield plasticly if the flange temperature exceeds the band temperature, thus resulting in a loosening or rupture of the band;
(2) it's reliability is inherently questionable since the integrity of the engine/aircraft system depends on a single, lonesome bolt; and
(3) the risk of failure of this single high stressed bolt is compounded by the fact that it may not be firmly seated and must be tensioned by a mechanical style turnbuckle.
A better band clamp is needed for reliable engine to exhaust system quick maintenance connection. Therefore, it is quite clear that a means is needed to quickly disconnect the engine from the exhaust system with access only from the bottom and to then quickly connect a replacement engine to the exhaust system.